(1) Field of the Invention
The present invention relates to a reference current circuit which generates a bias current provided to an analogue circuit.
(2) Description of the Related Art
FIG. 4 shows a structure of a conventional reference current circuit.
A reference current circuit 400 includes anon-inverting amplifier circuit 410 and a current source circuit 120.
The non-inverting amplifier circuit 410 is composed of an amplifier circuit OP40, a resistor R1, and a resistor R2. The amplifier circuit OP40 includes an inverting input terminal, a non-inverting input terminal, and an output terminal; the resistor R1 is inserted in a wiring connecting the inverting input terminal and a ground terminal; and the resistor R2 is inserted in a wiring connecting the output terminal and the inverting input terminal. The non-inverting input terminal of the amplifier circuit OP 40 receives input of a reference voltage VBG which is independent of a temperature T and a power supply voltage Vdd. In other words, the reference voltage VBG is temperature-compensated.
The current source circuit 120 is composed of a resistor R3, a transistor Q1, and a transistor Q2. One terminal of the resistor R3 is connected to the output terminal of the amplifier circuit OP40; a collector and a base of the transistor Q1 are connected to the other terminal of the resistor R3, while an emitter of the transistor Q1 is grounded; and a base of the transistor Q2 is connected to the collector and the base of the transistor Q1.
In general, a bandgap reference circuit is often used as a reference voltage circuit 500 which outputs the reference voltage VBG. FIG. 5 shows one example of such a bandgap reference circuit. The reference voltage circuit 500 is composed of an amplifier circuit OPS, a resistor R1a, a transistor Q1a, a resistor R2a, a resistor R3a, and a transistor Q2a. The resistor R1a is inserted in a wiring connecting an on-inverting input terminal and an output terminal of the amplifier circuit OPS; a base and a collector of the transistor Q1a are grounded; the resistor R2a is inserted in a wiring connecting an inverting input terminal and the output terminal of the amplifier circuit OPS; the resistor R3a is inserted in a wiring connecting the inverting input terminal and an emitter of the transistor Q2a; and a base and a collector of the transistor Q2a are grounded.
In the following, an outline of a principle which generates a reference current with low temperature dependence will be described in relation to the reference current circuit 400 structured as above.
The following equation holds for the non-inverting amplifier circuit 410.
                              V          BG                =                                            R              1                                                      R                1                            +                              R                2                                              ·                      V                          out              ⁢                                                          ⁢              4                                                          (        4.1        )            
The following equation is derived from the equation (4.1).
                              V                      out            ⁢                                                  ⁢            4                          =                              (                          1              +                                                R                  2                                                  R                  1                                                      )                    ·                      V            BG                                              (        4.2        )            
Here, VBG is a reference voltage which is independent of the temperature T and the power supply voltage Vdd; and R1 and R2 are resistances with a positive temperature coefficient. When the temperature is T, the following equations hold.
                    ∂                  V          BG                            ∂        T              =    0    ,                    ∂                  R          1                            ∂        T              =                            ∂                      R            2                                    ∂          T                    >      0      
Since the ratio of R1 and R2 remains the same regardless of temperature change, the equation (4.2) indicates that an output voltage Vout4 is temperature-independent.
Also, the following equation can be found for the current source circuit 120.Vout4=R3·Iref4+VBE  (4.3)
The equation (4.3) leads to the following equation.
                              I                      ref            ⁢                                                  ⁢            4                          =                                            V                              out                ⁢                                                                  ⁢                4                                      -                          V              BE                                            R            3                                              (        4.4        )            
By partially differentiating both sides of the equation (4.4) by the temperature T, the following equation can be obtained.
                                          ∂                          I                              ref                ⁢                                                                  ⁢                4                                                          ∂            T                          =                                            -                              1                                  R                  3                                                      ·                                          ∂                                  V                  BE                                                            ∂                T                                              -                                                                      V                                      out                    ⁢                                                                                  ⁢                    4                                                  -                                  V                  BE                                                            R                3                2                                      ·                                          ∂                                  R                  3                                                            ∂                T                                                                        (        4.5        )            
Also, based on the following inequality
            ∂              V        BE                    ∂      T        <  0
and “Vout4>VBE”, in the right side of the equation (4.5), the first term is positive, while the second term is negative. Thus, by adjusting each parameter such that the right side of the equation (4.5) becomes 0, a reference current Iref4 with low temperature-dependence is generated. This is how the temperature characteristic of the transistor Q1 is compensated by the temperature characteristic of the resistor R3.
However, in recent years, in the field of semiconductor integrated circuits, temperature dependences of resistances are becoming extremely low as miniaturization of resistors progresses. Also, there are limits to actual adjustment ranges when adjusting each parameter. Thus, with the above-mentioned conventional structure, it is becoming increasingly difficult to reduce temperature dependence of a reference current.
Non-Patent Document 1: Fundamentals of Analogue LSI Design, by Kajiro Watanabe and Tetsuo Nakamura, Ohmsha, 2006, pp. 149-151.